1. Field of the Invention
The invention relates to a new liquid crystal compound, a liquid crystal composition and a liquid crystal display device. The invention relates in particular to a five-ring liquid crystal compound having a wide temperature range of liquid crystal phases, a high clearing point, a good compatibility with other liquid crystal compounds, an appropriate optical anisotropy, and an appropriate dielectric anisotropy, and a composition comprising the compound, and a liquid crystal display device using the composition.
2. Related Art
Liquid crystal display devices using liquid crystal compounds have been widely applied to displays for watches, calculators, personal computers and so forth. In these display devices, the optical anisotropy, the dielectric anisotropy, and so forth of the liquid crystal compounds have been utilized.
In the liquid crystal display devices, a classification based on the operation mode of liquid crystals includes phase change (PC), twisted nematic (TN), super twisted nematic (STN), bistable twisted nematic (BTN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA), polymer sustained alignment (PSA) and so forth. A classification based on a driving mode includes a passive matrix (PM) and an active matrix (AM). The PM is further classified into static, multiplex, and so forth, and the AM is classified into a thin film transistor (TFT), metal insulator metal (MIM) and so forth.
A liquid crystal display device comprises a liquid crystal composition having suitable properties. The composition desirably has suitable properties for improving the characteristics of the device. General physical properties necessary for a liquid crystal compound which is a component of the composition are listed as follows.    (1) being chemically stable and physically stable,    (2) having a high clearing point (the phase transition temperature of a liquid crystal phase to an isotropic phase),    (3) being low in the minimum temperature of liquid crystal phases (a nematic phase, a smectic phase, and so forth), especially of the nematic phase,    (4) having an excellent compatibility with other liquid crystal compounds,    (5) having an appropriate optical anisotropy, and    (6) having an appropriate dielectric anisotropy.
A voltage holding ratio can be increased by using a composition containing a chemically and physically stable liquid crystal compound as described in item (1). The temperature range of a nematic phase is wide in a composition containing a liquid crystal compound having a high clearing point or a low minimum temperature of liquid crystal phases as described in items (2) and (3), and thus the device is usable in a wide temperature range.
The liquid crystal compound is generally used as a composition prepared by being mixed with many other liquid crystal compounds to obtain characteristics that cannot be attained with a single compound. Thus, it is desirable that the liquid crystal compound used for a display device has a good compatibility with other liquid crystal compounds and so forth, as described in item (4).
Recently, a liquid crystal display device has been required to have high quality in display performance such as contrast, display capacity, and response time. For example, a liquid crystal compound capable of decreasing the threshold voltage of its composition is required for decreasing the driving voltage of this device. It is desirable that the product (Δn×d) of the optical anisotropy (Δn) of the liquid crystal composition and the cell gap (d) of the liquid crystal display device is constant in order to attain a good liquid crystal display. Refer to E. Jakeman, et al., Phys. Lett., 39A, 69 (1972). Therefore, the liquid crystal compound is required to have the value of the optical anisotropy corresponding to the cell gap of the liquid crystal display device. Thus, the liquid crystal compound having an appropriate optical anisotropy as described in item (5) has been demanded.
Threshold voltage (Vth), as is well known, is represented by the following equation. Refer to H. J. Deuling, et al., Mol. Cryst. Liq. Cryst., 27 (1975) 81.Vth=π(K/∈0Δ∈)1/2 
In the equation above, K is an elastic constant and ∈0 is a dielectric constant in vacuum. As is shown by the equation, two methods are possible to decrease the threshold voltage Vth, either by increasing the value of the dielectric anisotropy (Δ∈) or by decreasing the value of the elastic constant (K). However, the elastic constant (K) is not easily controlled by the present technology, and a compound having a large dielectric anisotropy (Δ∈) is most commonly used to respond to the present demand. Under these circumstances, liquid crystal compounds having an appropriate dielectric anisotropy as described in item (6), and liquid crystal compounds having a large dielectric anisotropy have been developed actively.
Until now, liquid crystal compounds have been developed to attain an appropriate optical anisotropy corresponding to the cell gap of the liquid crystal display device and an appropriate dielectric anisotropy. However, only a few examples are reported concerning liquid crystal compounds having a relatively small optical anisotropy, a high clearing point, a good compatibility with other compounds, and an appropriate dielectric anisotropy.
A variety of liquid crystal compounds having a high clearing point and a large dielectric anisotropy have been synthesized until now and some of them are used practically. For example, four-ring-containing compounds having a CF2O-bonding group are disclosed in patent literatures Nos. 1 to 6. However, these compounds do not have a sufficiently high clearing point, so that the usable temperature range of their compositions are not sufficiently wide for the use of display devices.
Furthermore, five-ring-containing compounds (S-1) to (S-3) having a tetrahydropyran ring and the CF2O-bonding group are disclosed in patent literatures Nos. 7 to 12. These compounds do not have a sufficiently high clearing point. The compounds (S-4) and (S-5) having tetrahydropyran and dioxane rings are also disclosed in patent literature No. 11. However, the temperature range of liquid crystal phases is not sufficiently wide and the clearing points are not sufficiently high in these compounds.

The patent literatures cited are No. 1: WO 1996/11897 A, No. 2: JP H10-204016 A/1998, No. 3: GB 2229438 C, No. 4: DE 4023106 A; No. 5: JP H10-251186 A/1998, No. 6: WO 2004/035710 A, No. 7: WO2004/048501 A, No. 8: JP 2004-352721 A; No. 9: WO 2005/019378 A, No. 10: WO 2005/019381 A, No. 11: WO 2006/125511 A, and No. 12: WO2006/125530 A.